1. Field of the Invention
The present invention relates to an erbium-doped fiber amplifier used in a wavelength division multiplexing transmission system, and more particularly to an optical fiber amplifier, which is capable of controlling its gain to maintain the intensity of an output optical signal constant by driving a laser diode with a signal obtained by use of input optical signal filtering.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) is a method for transmitting a plurality of optical signals of different wavelengths via a single optical fiber. Since the WDM uses the different wavelengths of optical signals simultaneously when the optical signals are transmitted, a wide bandwidth provided by the optical fiber can be effectively utilized. Therefore, this method is being popularized as next generation optical transmission technology.
An erbium-doped fiber amplifier is a kind of amplifier used in a WDM transmission system. The erbium-doped fiber amplifier is manufactured by doping special material called erbium into the fiber. When erbium is pumped by a laser, a weak optical signal can be amplified by energy released when excited erbium ions return to their original energy level.
Now, a general erbium-doped fiber amplifier will be described with reference to the accompanying drawings.
FIG. 1 is a view showing a configuration of a general erbium-doped fiber amplifier.
Referring to FIG. 1, a first amplification stage comprises an erbium-doped fiber 123, optical couplers 121 and 122 connected to the front and rear of the fiber 123, and laser diodes 124 and 125 supplying excitation light, i.e., laser light for the optical couplers 121 and 122. Similarly, a second amplification stage comprises an erbium-doped fiber 153, optical couplers 151 and 152 connected to the front and rear of the fiber 153, and laser diodes 154 and 155 supplying excitation light for the optical couplers 151 and 152. A gain equalization filter 13 and an optical attenuator 14 are sequentially connected between the first and second amplification stages and a laser diode controller 16 produces a driving voltage required for each of laser diodes 124, 125, 154 and 155. The optical couplers 121 and 151 are forward IWDM (Isolation and Wavelength Division Multiplexing) optical couplers and the optical couplers 122 and 152 are backward IWDM optical couplers. Here, the term xe2x80x9cforwardxe2x80x9d means propagated in the same direction as an input optical signal, and the term xe2x80x9cbackwardxe2x80x9d means propagated in the opposite direction as an input optical signal.
In an amplification process in the first amplification stage, each of laser diodes 124 and 125 emits predetermined excitation light by the application of driving voltage supplied from the laser diode controller 16, and the excitation light is inputted to the erbium-doped fiber 123 by each of the optical couplers 121 and 122. The excitation light excites erbium ions included in the erbium-doped fiber 123. An optical signal inputted through an input port 11 and the optical coupler 121 can be amplified by energy released when erbium ions excited by the excitation light return to their original energy level. Such amplification is also achieved in the second amplification stage, and a resultant amplified optical signal is provided to the outside via an output port.
The gain equalization filter 13 located between the first and second amplification stages is for maintaining a balance of total gain by extracting a smooth portion of gain of the optical signal amplified in the first and second amplification stages. The optical attenuator 14 optimizes the optical signal by adjusting the intensity of the optical signal inputted into the second amplification stage.
In the above WDM transmission system, the number of channels of the optical signals is varied by capacity variation of the transmission network, errors of transmission channels, and any attachment and detachment of parts due to reconstruction of the transmission network. When the number of channels of the optical signals in use is varied, surviving channels in operation, i.e., remaining optical channels, move to an unwanted state through a transient state according to the characteristics of the erbium-doped fiber used as gain medium in the fiber amplifier, so the instantaneous change of gain and output power occurs, thus causing errors in optical transmission service.
Generally, since the fiber amplifier is constructed by connecting several amplification stages in series, though each of the amplification stages has a small variation of output, significant errors occur when the optical signal passes through the amplification stages in an optical transmission line. Therefore, there is a need to provide a gain control method for controlling the variation of output to be suppressed in a shorter time.
Generally, the erbium-doped fiber has gain inhomogeneity characteristics and cross gain saturation characteristics. The gain inhomogeneity characteristics mean a variation of gain of the surviving channels generated when the wavelength of the surviving channels having constant gain is varied. Gain in the erbium-doped fiber is shared by various optical channels, each having a constant gain value. The cross gain saturation characteristics mean equal distribution between the remaining optical channels generated when some of various optical channels are extinguished.
Therefore, since gain becomes varied according to wavelengths of the surviving channels and their distribution condition due to the gain inhomogeneity characteristics and the cross gain saturation characteristics of the erbium-doped fiber, there is a need to provide a gain control method for compensating for an output imbalance for each channel.
The gain control method of the fiber amplifier includes several methods that are described below.
A first method is to control the gain of the fiber amplifier by detecting input optical signals and adjusting excitation light such that it has a proper level of intensity.
However, although the above method is highest advantageous in terms of costs and operation, it has a problem that a range of control is widened in proportion to the number of channels used in optical transmission and it is required to provide a high speed excitation light control circuit having a higher response speed as the number of fiber amplifiers is increased in a system for remote transmission.
A second method is to control the gain of the fiber amplifier by adjusting the population inversion of the fiber amplifier by operating additional channels having wavelength bands different from those of multi-channel in operation. However, in this case, there is a problem that the additional channels require a high maximum output as the number of channels is increased, and a noise due to a nonlinear phenomenon occurs in the multi-channel optical signal in operation.
A third method is to control the gain of the fiber amplifier optically by inducing a laser emission through an optical feedback of some light outside the wavelength band of the multi-channel optical signals in operation so that the population inversion of the fiber amplifier is maintained. According to this method, the intensity of the fed-back laser-emitted light exhibits a damping oscillation in a transient variation state due to a variation of the intensity of the input optical signals. This damping oscillation is a phenomenon produced by an instantaneous perturbation of the population inversion in equilibrium if the upper-level lifetime of the erbium ions serving as a gain medium for the laser emission is longer than the lifetime of photons in a resonator. If this phenomenon is not removed or controlled to be less than a proper level, there occurs a problem that the surviving channels are badly affected. In addition, since this method requires a complicated circuit design in order to maintain the population inversion, there is a problem that high speed response characteristics can be not obtained.
Additionally, as a prior art for accomplishing an automatic gain control and an automatic light intensity control in the erbium-doped amplifier, disclosed is U.S. Pat. No. 6,055,092, issued on Jun. 25, 2000, entitled xe2x80x9cMulti-wavelength light amplifierxe2x80x9d. This patent proposes a two-stage amplification construction, each amplification stage performing a function of the automatic gain control by branching some of input and output optical signals and a function of the automatic light intensity control through an optical attenuator provided between the amplification stages after branching and detecting some of output of the second amplification stage. However, this patent does not provide a gain control method considering gain inhomogeneity characteristics peculiar to the erbium-doped fiber.
As another gain control method of the erbium-doped fiber amplifier, there is a method for re-circulating some of output so that a constant population inversion is maintained in the erbium-doped fiber, which is described in detail in xe2x80x9cGain-shifted EDFA with all-optical automatic gain controlxe2x80x9d, by M. Artiglia et al, ECOC ""98, pp 293-294, 1998. However, this paper proposes only performing a gain control by re-circulating some light, and, therefore, is not a gain control method considering the gain inhomogeneity characteristics of the erbium-doped fiber in an excitation light control method.
In other words, by any of the above-described prior art methods, the problems of gain imbalance between the multi-channel optical signals and the intensity difference of light due to the gain inhomogeneity characteristics and cross gain saturation characteristics of erbium-doped fiber could not be overcome.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an erbium-doped fiber amplifier capable of controlling its gain so that the intensity of light of surviving channels outputted from the fiber amplifier is constantly maintained in a shorter time by compensating for gain inhomogeneity characteristics through a control of a laser diode based on a signal obtained using an input optical signal filtering when a gain control of an excitation light control system is applied.
In order to accomplish the above object, the present invention provides an erbium-doped optical fiber amplifier using input optical signal filtering, comprising: a first amplification stage for receiving an input optical signal and forward and backward excitation light, and amplifying the input optical signal using energy released when erbium ions excited by the excitation light return to their original energy level; a pre-processing means for performing a gain equalization process and a light intensity attenuating process for the signal amplified in the first amplification stage; a second amplification stage for receiving the optical signal processed in the pre-processing means and the forward and backward excitation light and performing a second amplification operation; a plurality of laser diodes for generating the forward and backward excitation light and providing it to the first and second amplification stages, respectively; an input optical signal detector for branching and receiving some of the input optical signal, separating the received optical signal into two signals having a ratio of 50:50, and photo-electrically converting a first one of the separated optical signals by performing a filtering process for compensating for the gain inhomogeneity characteristics of the fiber, and a second one without any filtering; and a laser diode controller for generating forward diode control voltage of the first amplification stage using the filtered signal of the electrical signals provided by the input optical signal detector and generating other laser diode control voltage using the signal that is not filtered.